magento2 – Where do scalar extension attributes save to?

Been reading the doc from Magento and i have been using extension attributes for more complex data structures but i’m basically wanting to add a simple scalar attribute

  <extension_attributes for="MagentoSalesApiDataShipmentTrackInterface">
    <attribute code="track_url" type="string"/>
  </extension_attributes>

And i thought for scalar attributes, i pretty much don’t need to do anything else such as adding plugins, just need to set the value correctly and use repository to save

        $shipment = $this->shipmentRepository->get($shipmentId);

        $track = $this->trackFactory->create()
            ->setTrackNumber('123')
            ->setCarrierCode('abc')
            ->setTitle('super delivery');


        $extensionAttributes = $track->getExtensionAttributes()->setTrackUrl(
            'http://www.trackme.com/abcdef'
        );
        $track->setExtensionAttributes($extensionAttributes);
      

        $shipment->addTrack($track);
        $this->shipmentRepository->save($shipment);

Codes are generated correctly, no errors, but when i try to retrieve it, it doesn’t actually return anything

$track->getExtensionAttributes()->getTrackUrl()

Am i misunderstanding extension attributes then? Does it mean i will always need to add something new to the database, even for scalar types?